In general, conventional aqueous based latex topcoats require using a primer undercoat to provide adequate adhesion and tannin blocking to wood and previously painted surfaces. Tannin blocking is the ability of a coating to prevent water soluble chromophoric compounds, present in or on a substrate or substrate coating, from migrating through a newly applied topcoat. For example, certain woods such as redwood and cedar contain tannin staining agents. If a conventional latex coating is applied to the wood, these staining agents become extracted from the wood and migrate through the coating. Repeated applications of the same or a similar type of aqueous latex coating will not successfully prevent these stains from reappearing on the new coating surface. Solvent based coatings are typically effective in blocking these stains since the stains are not solubilized by the coating solvent.
While there are certain benefits to using a solvent based coating, aqueous coatings are preferred over solvent based coatings for many reasons. Reduction of the VOC of the coating is an advantage offered by latex coatings. In addition, alkyd coatings are not generally as crack, fade and flake resistant as latex based coatings, particularly in exterior applications. Specifically designed latex based coatings may have better adhesion to substrates such as galvanized steel and cement based substrates. Latex based coatings also offer easy water clean-up.
The latex based primers currently available provide adequate stain blocking, but in order to obtain the necessary durability and attractive appearance for exterior applications, it is recommended that the latex based primer be coated with two additional layers of latex paint. The latex primer improves adhesion to the substrate and helps to insure a uniform appearance of the top coat. Primers must be topcoated because they are discolored by the tannins contained in the substrate. The latex primers function by letting the tannins migrate into the primer and then trapping the tannins in the primer. This prevents subsequent latex layers from being discolored by the tannins. The additional layers of latex paint provide the durability to withstand weathering and provide a uniform, attractive appearance to the resulting coated substrate. It is desirable, however, to obtain the stain blocking properties and durability of a three layer latex coating system in a single layer of latex based coating.
The present invention is directed to a latex paint which does not require a primer for application over wood, masonry, metal, vinyl siding and previously painted surfaces. The latex paint of the present invention can be applied in a single coat directly over these surfaces to provide the tannin blocking, hiding, corrosion resistance and durability of a conventional primer plus two coats of latex paint. This primerless paint is useful for both interior and exterior applications.